1. Field of the Invention
The present invention relates to a development device used in image forming apparatuses such as a copy machine, a printer, a facsimile, and the like that form an image by an electrophotographic system.
2. Related Background Art
Heretofore, in image forming apparatuses such as a copy machine, a printer, a facsimile, and the like that form a color image by an electrophotographic system, there is known a multi-transfer system which includes yellow, magenta, cyan, and black development units, sequentially overlaps and transfers the respective color toner images, which are developed on an image bearing member such as a photosensitive drum, and the like, onto a transfer member or sequentially overlaps and transfers them onto an intermediate transfer member, and forms a full-color image by transferring them onto an image transfer member at once.
In the image forming apparatus, when the toner in the development units is exhausted, fresh toner is supplied (replenished) to the development units from toner cartridges (toner replenishment vessels) in which the fresh toner is accommodated.
In for example, a technology disclosed in Japanese Patent Application Laid-Open No. 10-149012, toner is replenished to development units from respective toner cartridges through toner transport pipes (toner replenishment paths), which are interposed between the development units and the toner cartridges, and toner transport screws (supply augers) disposed in the toner transport pipes.
An image forming operation executed by the technology disclosed in Japanese Patent Application Laid-Open No. 10-149012 will be explained using FIGS. 7 and 8.
FIG. 7 is a side sectional view showing a schematic arrangement of a conventional rotary development device. In FIG. 7, the conventional rotary development device 50 includes a plurality of development units 52 (yellow, magenta, cyan, and black development units in FIG. 7,) mounted on a rotatable support frame 51, a plurality of toner replenishment vessels 53 mounted on the support frame 51 adjacent to the development units 52, toner transport pipes 55, which are disposed approximately in parallel with a rotary shaft 54 of the support frame 51 and causes the development units 52 to communicate and couple with the toner replenishment vessels 53 adjacent to the development units 52, and toner transport screws 56 for transporting fresh toner in the toner transport pipes 55 from the toner replenishment vessels 53 to the development units 52.
FIG. 8 is a plan view explaining a lengthwise arrangement of the conventional rotary development device 50. After the fresh toner accommodated in the toner replenishment vessels 53 is transported into the toner transport pipes 55 by toner transport means 57 in the toner replenishment vessels 53, it is supplied to the respective development units 52 by the toner transport screws 56 in the toner transport pipes 55.
In the arrangement in which the development units 52 are disposed approximately in parallel with the rotary shaft 54 of the rotatable support frame 51 (hereinafter, referred to as a rotary development system), a phenomenon occurs in that the toner in the toner transport pipes 55 is shifted by the rotation (hereinafter, referred to as revolution) of the support frame 51 for switching the development units 52 with respect to an image bearing member 58. That is, when the rotary development device 50 is revolved to switch the development units, there is a possibility that toner is excessively replenished by the toner transport screws 56 in the toner transport pipes 55 or toner flows back into the toner replenishment vessels 53 depending on the winding direction of the toner transport screws 56. When toner is excessively replenished to the development units 52 or is caused to flow back into the toner replenishment vessels 53, it is unstably replenished to the development units 52.
To cope with the above problem, in the technology disclosed in Japanese Patent Application Laid-Open No. 10-149012, the winding direction of the toner transport screws 56 is arranged such that when the rotary development device 50 is revolved to switch the development units, the toner shifts in the direction from the development units 52 to the toner replenishment vessels 53.
The toner transport screws 56 whose winding direction is arranged to shift the toner in the direction from the development units 52 to the toner replenishment vessels 53 is effective to prevent the excessive replenishment of the toner to the development units 52 when the rotary development device 50 is revolved as described above. However, there is a possibility that a toner replenishing time is increased or an amount of replenished toner is dispersed.
More specifically, in the arrangement for replenishing toner disclosed in Japanese Patent Application Laid-Open No. 10-149012, when the toner transport screws 56 begin to rotate to replenish toner to the development units 52, the toner in the vicinity of toner inlets of the development units 52 is transported to the development units 52. However, toner is transported from the development units 52 to the toner replenishment vessels 53 at all times when the rotary development device is revolved as described above even if the rotation of the toner transport screws 56 is stopped, thereby the amount of toner in the vicinity of the toner inlets is reduced. Accordingly, when toner is replenished, the rotating time of the toner transport screws 56 must be increased in correspondence with the amount of reduced toner.
Further, since the amount of toner shifted in the toner transport pipes 55 by the revolution of the rotary development device described above is also affected by the amount of toner in the toner replenishment vessels 53, toner cannot be replenished to the development units 52 in a predetermined amount. Accordingly, there is a possibility that the amount of replenished toner is dispersed. When, for example, the amount of toner in the toner replenishment vessels 53 is reduced, toner is liable to shift in the revolution and thus the toner in the vicinity of the toner inlets is transported to the development units 52, thereby the amount of toner in the vicinity of the toner inlets is reduced. In contrast, when a large amount of toner is accommodated in the toner replenishment vessels 53, a small amount of toner is shifted. As a result, since the amount of toner replenished to the development units 52 is dispersed due to the amount of toner in the toner replenishment vessels 53, an image is unstably developed on the image bearing member 58 by the development units 52 and thus an image density is also made unstable, which adversely affects an output image.